Generally, with regard to vessels in ballast or with little loads, water in the port (seawater, lake water, or river water) is taken in the vessels as ballast water before leaving the port, because it is necessary to secure a propeller submergence depth and safe navigation in ballast, etc. Conversely, when cargo is loaded in a vessel loaded with ballast water in the ballast tank in the port, the ballast water is discharged from the ballast tank. In the meantime, if ballast water is taken in and discharged from a vessel reciprocating between a loading port and an unloading port of different environments, the living organisms contained in the water of one port are discharged from the vessel at the other port along with the discharged ballast water. Therefore, there is concern that the coastal ecosystem of the other port may be adversely affected by the discharge. Under the circumstances, at the International Conference on Ballast Water Management of Ships, the International Convention for the Control and Management of Ship's Ballast Water and Sediments was adopted in February 2004, and it became obligatory to treat living organisms in ballast water according to the International Convention.
The standard defined by the International Maritime Organization (IMO) for the treatment of living organisms in ballast water (IMO ballast water treatment standard) stipulates that the number of living organisms (mainly, zooplankton) of 50 μm or more in size contained in ballast water discharged from a vessel is less than ten in 1 m3, the number of living organisms (mainly, phytoplankton) of 10 μm or more and 50 μm or less in size is less than ten in 1 ml, the number of vibrio cholerae is less than 1 cfu in 100 ml, the number of E. coli is less than 250 cfu in 100 ml, and the number of enterococci is less than 100 cfu in 100 ml.
Japanese Patent Application KOKAI Publication No. 2007-144391 (Patent Literature 1) and JFE Engineering Co., Ltd., Ballast Water Management System for Ships, [online], [Search on Apr. 13, 2016], Internet <URL: http://www.jfe-eng.co.jp/products/comfortable/marine/mar01.html> (Non-Patent Literature 1) each discloses a ballast water treatment device including a filtering device for filtering seawater as raw water for ballast water and capturing aquatic organisms in the seawater, and a disinfectant supply device for supplying an oxidant as a disinfectant into the filtered seawater to kill bacteria in the seawater.
In the ballast water treatment device disclosed in each of Patent Literature 1 and Non-Patent Literature 1, sodium hypochlorite or the like is used as a disinfectant, and the supply amount of the disinfectant is adjusted so that the concentration (chlorine concentration) of the disinfectant in the seawater injected into the ballast tank is kept at a predetermined appropriate concentration (constant value).
For example, in the ballast water treatment device of Non-Patent Literature 1, the supply amount of the disinfectant is usually adjusted as follows. First, the disinfectant concentration in seawater that is deemed appropriate is determined in advance. Next, while receiving the supply of the disinfectant, the concentration of the disinfectant in the seawater to be injected into the ballast tank is measured. Thereafter, the measured disinfectant concentration is compared with the predetermined disinfectant concentration, and it is determined whether the measured disinfectant concentration is more than or less than the predetermined disinfectant concentration. Then, by feedback control based on the determination result, output adjustment of a pump for supplying the disinfectant and adjustment of the opening degree of a valve provided in a disinfectant supply line are performed, thereby adjusting the supply amount of the disinfectant. Usually, the predetermined disinfectant concentration has a predetermined acceptable range. In this case, the amount of the disinfectant supplied as described above is adjusted so that the measured concentration of the disinfectant falls within this predetermined range (hereinafter also referred to as “target concentration range”).
The concentration of the disinfectant in the seawater stored in the ballast tank after the disinfectant is supplied decreases with the lapse of time due to the reaction between the disinfectant and the organic matter in the seawater and the self-decomposition reaction of the disinfectant. As a result, there is a possibility that living organisms may regrow and plankton eggs may hatch in the seawater stored in the ballast tank. Therefore, the concentration of the residual disinfectant in the stored seawater (the concentration of the disinfectant remaining in the stored seawater after the lapse of a predetermined time) must be maintained to the extent that the regrowth of the aquatic organisms and the hatching of the plankton eggs can be suppressed.
The content of organic matter (water quality) in water to be used as ballast water (seawater, lake water, or river water, hereinafter collectively referred to as “raw water” in this specification) greatly varies depending on the port of call of the vessel. The extent of the reaction between the disinfectant and the organic matter in the raw water, in other words, the degree of decrease in the concentration of the disinfectant in the raw water over time, depends on the quality of the raw water. Therefore, conventionally, the target concentration range of the above-mentioned disinfectant concentration is set within a wide range higher than the disinfectant concentration set for standard water-quality raw water so that a sufficient residual disinfectant concentration can be maintained for raw water of various water quality, and the amount of the disinfectant supplied to the raw water is adjusted so that the concentration of the disinfectant in the raw water injected into the ballast tank falls within the target concentration range.